We had the same problem with IIS 6 on a server using Citrix that had the default port 80 reserved.
The easiest way we found to resolve this issue is to:

  • Open a command prompt and type netstat -ano ← This will show you currently reserved ports, you should see port 80, among others. Take note of these, you cannot use them as an alternative for your Default Website
  • Go to IIS manager. Right click on the Default Website, go to Properties
  • You can edit the TCP port (something like 8080 should do just fine if not already reserved) and apply the changes.
  • Try to start the Default Website again

Now, to access your SBC, you just need to add the port to the url,like http://servername.domain:8080/avast/service.aspx

Hope this works for you.